Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. The thrust faults responsible for megathrust earthquakes often lie at the bottom of oceanic trenches; in such cases, the earthquakes can abruptly displace the sea floor over a large area. As a result, megathrust earthquakes often generate tsunamis that are considerably more destructive than the earthquakes themselves. Teletsunamis can cross ocean basins to devastate areas far from the original earthquake.
Terminology and mechanism
The term megathrust refers to an extremely large thrust fault, typically formed at the plate interface along a subduction zone, such as the Sunda megathrust. However, the term is also occasionally applied to large thrust faults in continental collision zones, such as the Himalayan megathrust. A megathrust fault can be 1,000 kilometers (600 mi) long.
A thrust fault is a type of reverse fault, in which the rock above the fault is displaced upwards relative to the rock below the fault. This distinguishes reverse faults from normal faults, where the rock above the fault is displaced downwards, or strike-slip faults, where the rock on one side of the fault is displaced horizontally with respect to the other side. Thrust faults are distinguished from other reverse faults because they dip at a relatively shallow angle, typically less than 45°, and show large displacements. In effect, the rocks above the fault have been thrust over the rocks below the fault. Thrust faults are characteristic of areas where the Earth's crust is being compressed by tectonic forces. Megathrust faults occur where two tectonic plates collide. When one of the plates is composed of oceanic lithosphere, it dives beneath the other plate (called the overriding plate) and sinks into the Earth's mantle as a slab. The contact between the colliding plates is the megathrust fault, where the rock of the overriding plate is displaced upwards relative to the rock of the descending slab. Friction along the megathrust fault can lock the plates together, and the subduction forces then build up strain in the two plates. A megathrust earthquake takes place when the fault ruptures, allowing the plates to abruptly move past each other to release the accumulated strain energy.
Occurrence and characteristics
… excerpt ends here. Continue reading the full article.
